1. Field of the Invention
The present invention relates to an imaging device such as a digital camera, a method for controlling an imaging device and a program product, and particularly to an imaging device, a method for controlling an imaging device and a program product which can perform auto exposure control.
2. Description of Related Art
In general, when an exposure control of a CCD (Charge Coupled Device) or a CMOS (Complementary Metal Oxide Semiconductor) is performed, it is necessary to control both of “an exposure time (shutter speed)” and “a gain” in sets (for example, Japanese Unexamined Patent Application Publication No. 2000-78461). It is important that these two control information are controlled with different timings with respect to one image data. In a normal exposure control, “an exposure time” is required to be determined and set first. Then two separate processing including a first stage processing and a second stage processing are executed. In the first stage processing, a light receiving element stores charge in response to incident light according to the exposure time that is determined and outputs the cumulative charge (luminance). In the second stage processing, an exposure is controlled by gain (sensitizing) (post-processing) which compensates for deficiency to get an appropriate exposure with respect to this output. That is, an appropriate exposure is obtained by “incident light×exposure time×gain”. To achieve this in a digital camera, it is necessary that three steps which are “setting exposure time”, “exposure (light accumulation)”, and “gain (sensitizing)” are performed in series. These processing cannot be conducted in parallel with respect to one image data.
Therefore, when there is no information of exterior (incident light) in advance, for example, when a camera is booted up or scenes are greatly changed, the following processing is necessary to set an appropriate exposure value according to the situation of the exterior. Firstly, appropriate “exposure time” and “gain” are set, an obtained image data (luminance of an image=incident light×exposure time×gain) is evaluated, and the evaluated result is fed back to the next exposure time and gain. An appropriate exposure cannot be obtained without execution of these processing. To perform these processing, it is necessary to evaluate an extensive image (whole image) which is obtained by the first stage. The image data is accumulated once, and after finishing the evaluation, a gain processing needs to be performed on the whole image again. This leads to an increasing circuit size and a delaying processing time and there is a problem that it takes a long time to stabilize control exposure of a camera.
By the way, in Japanese Unexamined Patent Application Publication No. 2006-154642, an exposure control is performed based on a feedback by fill light of a passive AF (passive automatic focus) at the time of start-up. However, a camera which does not mount a special device which can measure lightness by floodlight and light-receiving such as a passive AF in a side of camera cannot be applied. Although some expensive digital camera may mount the device, it is often difficult to mount the device in a system for a mobile phone and so on due to problems such as a price, a size, and an electrical power.